When a cable is to be placed inside of a conduit or other passage, particularly when the conduit or passage is already installed in a building or other structure, it is common to use a pulling wire which is smaller than the cable to be installed and which can be metallic or non-metallic. Commonly, the cable includes a number of electrical conductors or, more recently, optical fibers and has a significant diameter relative to the size of the passage. The smaller pulling wire is first inserted and then is coupled to an end of the cable so that the cable can be pulled through the passage.
The apparatus for coupling the wire to the cable can take various forms, depending on the nature of the cable. One device which is quite useful is a mesh sleeve which radially contracts when it is subjected to longitudinal tension. This sleeve is placed over the end of the cable in a radially expanded state and is then elongated so that it contracts around the cable, gripping the cable more firmly as axial tension increases.
Sleeves of this type, as well as other kinds of gripping devices, are often provided with a coupling device at the end which is connected to the end of the sleeve either detachably or permanently, and which is connectable to the pulling wire. This coupling is intended to perform two basic functions, one being to transfer the longitudinal tension forces from the pulling wire to the sleeve and cable. The other function is to prevent harmful twisting torques from building up in the wire and cable. In order to perform this latter function, the coupling device is often made so that one part is rotatable relative to the other and is then referred as a swivel coupling.
Swivel couplings for various purposes are shown in the following patents:
U.S. Pat. No. 2,387,599 Miller et al, PA1 U.S. Pat. No. 2,651,533 Miller
It has been found, however, that the requirements for pulling fiber optic cables are such that conventional swivel couplings are not suitable, partly because they are generally somewhat too large, or because they do not have sufficient rotational freedom. If made smaller with bearings such as those in the above patents, such devices are very expensive and are not capable of withstanding some of the forces imposed, but if the bearings are eliminated the torques which develop are greater than can be safely imposed on the cable and other related hardware.